Handle dynamic field properties

I found a situation where gdb could not properly decode an Ada type.
In this first scenario, the discriminant of a type is a bit-field.
PROP_ADDR_OFFSET does not handle this situation, because it only
allows an offset -- not a bit-size.

My original approach to this just added a bit size as well, but after
some discussion with Eric Botcazou, we found another failing case: a
tagged type can have a second discriminant that appears at a variable
offset.

So, this patch changes this code to accept a general 'struct field'
instead of trying to replicate the field-finding machinery by itself.

This is handled at property-evaluation time by simply using a 'field'
and resolving its dynamic properties.  Then the usual field-extraction
function is called to get the value.

Because the baton now just holds a field, I renamed PROP_ADDR_OFFSET
to PROP_FIELD.

The DWARF reader now defers filling in the property baton until the
fields have been attached to the type.

Finally, I noticed that if the discriminant field has a biased
representation, then unpack_field_as_long would not handle this
either.  This bug is also fixed here, and the test case checks this.

Regression tested on x86-64 Fedora 41.

2 files changed, 112 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.ada/packed_record_2.exp b/gdb/testsuite/gdb.ada/packed_record_2.exp
new file mode 100644
index 0000000..d0bcdbd
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/packed_record_2.exp
@@ -0,0 +1,61 @@
+# Copyright 2025 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib "ada.exp"
+
+require allow_ada_tests
+
+standard_ada_testfile exam
+
+set flags {debug}
+if {[ada_minimal_encodings]} {
+    lappend flags additional_flags=-fgnat-encodings=minimal
+}
+
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable $flags] != ""} {
+    return -1
+}
+
+clean_restart ${testfile}
+
+set bp_location [gdb_get_line_number "STOP" ${testdir}/exam.adb]
+runto "exam.adb:$bp_location"
+
+set spr_contents "discr => 3, field => -4, array_field => \\(-5, -6, -7\\)"
+
+gdb_test "print spr" " = \\($spr_contents\\)"
+
+gdb_test "print spr.discr" " = 3"
+
+# See PR ada/32880 -- gdb should probably print array (1 .. 3) here,
+# but instead shows array (<>).  However as this isn't totally
+# relevant to this test, we just accept it.
+gdb_test "ptype spr" \
+    [multi_line \
+	 "type = tagged record" \
+	 "    discr: range 1 .. 8;" \
+	 "    field: range -7 .. -4;" \
+	 "    array_field: array \\(<>\\) of exam.small <packed: 2-bit elements>;" \
+	 "end record"]
+
+gdb_test_multiple "print sc" "" {
+    -re " \\($spr_contents, outer => 2, another_array => \\(-7, -6\\)\\)" {
+	pass $gdb_test_name
+    }
+    -re " \\($spr_contents, outer => $decimal, another_array => \\(.*\\)\\)" {
+	# Other output is a known GCC bug.
+	xfail $gdb_test_name
+    }
+}
diff --git a/gdb/testsuite/gdb.ada/packed_record_2/exam.adb b/gdb/testsuite/gdb.ada/packed_record_2/exam.adb
new file mode 100644
index 0000000..e528ecf
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/packed_record_2/exam.adb
@@ -0,0 +1,51 @@
+--  Copyright 2025 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+procedure Exam is
+   type Small is range -7 .. -4;
+   for Small'Size use 2;
+
+   type Range_Int is range 1 .. 8;
+   for Range_Int'Size use 3;
+
+   type Packed_Array is array (Range_Int range <>) of Small;
+   pragma pack (Packed_Array);
+
+   type Some_Packed_Record (Discr : Range_Int) is tagged record
+      Field: Small;
+      Array_Field : Packed_Array (1 .. Discr);
+   end record;
+   pragma Pack (Some_Packed_Record);
+
+   type Sub_Class (Inner, Outer : Range_Int)
+   is new Some_Packed_Record (Inner) with
+      record
+         Another_Array : Packed_Array (1 .. Outer);
+      end record;
+   pragma Pack (Sub_Class);
+
+   SPR : Some_Packed_Record := (Discr => 3,
+                                Field => -4,
+                                Array_Field => (-5, -6, -7));
+
+   SC : Sub_Class := (Inner => 3,
+                      Outer => 2,
+                      Field => -4,
+                      Array_Field => (-5, -6, -7),
+                      Another_Array => (-7, -6));
+
+begin
+   null;                        --  STOP
+end Exam;